The present invention relates to improvements made to sealed electrical terminals, also called in the art bulkhead fittings or bushings. These bushings are commonly used for feeding a high-intensity medium-voltage current to large electrical machines, and especially to the motors of high- and medium-power refrigerating units.
In the particular application to refrigerating units, the bushings must be absolutely sealed since the motors of these units are cooled by a fraction of the refrigerant, such as freon or a chlorofluorocarbon, and they must therefore bar the way to these fluids which are excessively quick to escape.
The sealed bushings of the prior art generally comprise a cylindrical central rod made of metal, such as copper, provided at its ends with threaded portions, to which may be fastened, respectively, the end of a cable coming from a power supply and the end of the coil of the motor which is located inside the metal casing of the motor, at least one insulating tubular body, for example made of ceramic, fitted around the central rod, and a metal tubular cap which is slipped around the insulating body or bodies and is crimped onto this or these insulating bodies, at least one seal being provided in order to seal between the insulating body or bodies and the cap.
The body has, on its outer wall, a threaded portion by means of which the bushing can be screwed into the wall of the casing of the motor and a portion in the form of a nut for driving the bushing when screwing it into the wall.
There are currently on the market two types of sealed bushings: bushings with two insulating bodies and bushings with a single insulating body.
1. In the bushings of the first type, the two insulating bodies are slipped around the central rod, respectively from the ends thereof, an insert seal of cylindrical shape and made of a deformable material, generally Teflon, being interposed between the adjacent ends of the two insulating bodies. The two other ends of the rod project from the ends of the insulating bodies. Each of the insulating bodies has a cylindrical head portion of larger diameter and a cylindrical shank portion of smaller diameter, these two portions being joined together by an annular shoulder. As regards the cap, this has a stepped inner wall complementary to that of the two insulating bodies and it terminates in a tubular crimping portion which can be crimped onto these bodies. Under the action of the axial compressive force that the two insulating bodies exert on the Teflon seal when crimping the cap, the insert seal expands radially so that its inner and outer cylindrical walls are applied very intimately against the outer wall of the central rod and against the inner wall of the cap. Thus, excellent sealing is achieved, on the one hand, between the insulating bodies and the central rod and, on the other hand, between the insulating bodies and the cap.
2. In the bushings of the second type, the single insulating body has a cylindrical portion of large diameter, to which an O-ring seal is fastened, and, on each side of said cylindrical portion of large diameter, two cylindrical portions of smaller diameter. In this type of bushing, the central rod projects from only one end of the insulating body, while its other end is housed at the bottom of a deep recess formed in the other end of the insulating body. The cap has the same structure as in the previous case.
A sealed bushing, whether of the first or of the second type described above, is fastened to the metal casing of the motor by screwing the cap into a tapped hole in the casing and then by connecting the ends of the supply cable and of the coil of the motor to the threaded ends of the central rod. For this purpose, the cable and the coil are provided at their free end with lugs which are clamped tightly between two nuts screwed onto the threaded portions of the central rod.
It is when exerting a clamping torque on the nuts, on the outside as on the inside of the casing, that the risk arises of the central rod starting to rotate with respect to the cap, thereby entraining with it the insulating body or bodies and the seal or seals integral with the rod. It is true that this inopportune rotation of the central components of the bushing in the cap does not necessarily impair the sealing of the bushing, since the seals remain in tight contact with the surface of the central rod and with the inner surface of the cap, but it represents a source of worry to the operator responsible for the connection, who wonders whether he has destroyed the essential absolute sealing characteristics of the bushing.
The aim of the present invention is to remedy this drawback and the object of the invention is to provide both types of bushings of the prior art with a certain number of improvements by means of which the central rod, and the insulating body or bodies and the seal or seals which are integral therewith, are prevented from rotating inside the cap, even if an inordinate torque is applied to the nuts for clamping the lugs.
For this purpose, the present invention provides a sealed bulkhead bushing of the type described above, which is characterized in that the central rod, the insulating tubular body or bodies, the seal and the cap have, on their mutual contact regions, components or portions having complementary reliefs, which are discontinuous or irregular, in the circumferential direction, said reliefs being capable of fitting one into another when crimping the cap onto the tubular insulating body or bodies, thus rotationally locking the latter with respect to the cap and to the central rod.
The reliefs which prevent the cap from rotating with respect to the insulating tubular bodies may be made in various ways.
According to one particular embodiment of the invention, applied to the first type of bushing described above, these reliefs consist of notches formed in that inner wall of the cap which is in contact with the cylindrical wall of the insert seal.
When crimping the cap, the material of the insert seal creeps and fills said notches, so that ribs of complementary shape to that of the notches are formed on the cylindrical wall of the insert seal. These ribs, when engaged with the notches, prevent the central rod and the seal from rotating with respect to the cap.
According to a second embodiment of the invention, applied to the second type of bushing described above, the anti-rotation locking is obtained by means of the combination of the following characteristics:
between the central portion of larger outside diameter of the insulating body and one of the end portions of smaller outside diameter of the insulating body there is a cylindrical bearing surface of diameter lying between that of the central portion of larger diameter and that of said end portion;
the cap has a stepped inner profile of complementary shape to that of the insulating body and includes, in particular, a bore of intermediate diameter which fits around the cylindrical bearing surface of the insulating body;
provided on the cylindrical bearing surface of the insulating body is at least one recess into which is partially inserted a removable locking component, part of which projects radially out of the recess without, however, going beyond the level of the cylindrical wall of the central portion of larger outside diameter;
said bore of the cap has at least one U-shaped longitudinal notch of depth at least equal to the height by which the locking component projects from the recess.
The locking component may consist of a ball or of a key. In the first case, the recess consists of a hemispherical blind hole which houses half of the volume of the ball, and in the second case, the recess consists of a longitudinal slot extending over the entire length of the cylindrical bearing surface.
By means of this arrangement, the cap can be slipped around the insulating body without the projecting portion of the ball or of the key scoring the inner surface of the cap.
As explained above, in a sealed bushing according to the first type, the axial and rotational locking of the central rod with respect to the insulating tubular bodies is achieved by clamping the insert seal against the central rod. This locking may be reinforced in various ways. For example, the central rod may include two diametrically opposed projections which act as axial stops and which are housed in notches formed in the inner wall of the insulating tubular bodies. The central rod may also include knurled portions on the parts which lie inside the tubular bodies and, in addition, the rod portions which lie inside the tubular bodies, including the knurled portions, may be coated with adhesive.
These various methods of anchoring the insulating bodies to the central rod, which use knurlings, projections and adhesive, have a certain number of drawbacks. Thus, if the bulkhead bushing is exposed to a relatively high temperature, the metal rod expands greatly. Given that the gap between the rod and the tubular insulating body is filled with adhesive, there is no longer any space available to absorb this expansion. The tubular insulating body therefore fully takes the radial rod expansion forces and ends up splitting.
In addition, the transfer of the torsional couple of the metal rod to the tubular insulating body is quite poorly provided by the projections and by the adhesive, given that a function of said projections is merely to act as a stop, ensuring that the bonding is carried out up to the desired height.
Furthermore, the adhesive may deteriorate over time, naturally or due to the action of chemicals.
The invention provides another way of anchoring the insulating bodies to the central rod, which requires neither the formation of projections and knurling, nor the use of adhesive.
According to the invention, the anchoring is provided by at least one removable linking component having a portion which is inserted into a housing cut into the metal rod and a portion which is inserted into at least one axial housing formed in the inner wall of the tubular insulating body.
Several linking components may be used to provide this anchoring, for example two diametrically opposed linking components on the metal rod, or three linking components arranged at 120xc2x0 one with respect to another, or even four linking components arranged at 90xc2x0 one with respect to another, etc.
Each linking component may consist of a ball, a pin or a key.
By using the linking components, it is even possible to leave a non-zero gap free between the central rod and the tubular insulating bodies in order to allow the rod to expand freely when the bushing is exposed to a high temperature. In this case, the central rod will exert a radially outward thrust on the insulating bodies only at the points of contact of the linking components with the insulating bodies. These points of contact are limited in number (one, two, three or four) and lie precisely in regions in which the wall of the insulating bodies is the thickest and not over the entire length of the insulating body as was the case when the rod was adhesively bonded to the insulating bodies. Destruction of the insulating bodies is thus avoided.
Another advantage stems from the fact that the torsional moment obtained using a few linking components is much higher than that obtained with the knurling and adhesive bonding (1.5 to 2.5 times greater).